1. Field of the Invention
The present invention generally relates to radar systems, and more particularly, to methods and apparatuses for stabilizing the automatic gain control (AGC) of a radar receiver and processing radar signals reflected from a target.
2. Description of the Related Art
In World War II, radar systems using centimeter wavelengths were discovered to detect reflections from precipitation, such as rain and hail. Since this discovery, radar systems have developed into extremely useful tools for identifying and classifying weather systems, particularly in the 3 cm to 10 cm wavelength range. For example, a weather radar system is typically installed on an aircraft to identify the conditions within a weather system, such as a cloud, that the aircraft is approaching. Radar waves emitted from the aircraft's radar system reflect from precipitation in the cloud and return to the aircraft. The radar system detects and analyzes the reflected waves to identify the range and characteristics of the cloud. In particular, the power of the reflected signal is proportional to the cloud's reflectivity, which is in turn related to the amount of rainfall within the cloud. Consequently, by timing the delay between the pulse emission and detection and comparing the power of the reflected signal to a series of power thresholds, the range to and characteristics of the cloud may be established.
As the range to the target increases, however, the signal-to-noise ratio drops dramatically. Like any light waves, the intensity of a radar pulse fades in a manner proportional to the square of the distance propagated. The waves are further attenuated due to atmospheric loss and the like. As a result, the signal from distant targets is relatively weak, so that background noise and receiver generated noise, which occurs in every environment, tend to obscure the reflected signal. To extract useful information from distant target reflections, amplification and processing of the detected reflections are consequently necessary to compensate for the lower signal-to-noise ratio. In addition, it is desirable to reduce the receiver generated noise as much as possible to ensure that the displayed signal is accurate and not unduly obscured by such noise (also referred to in this application as false alarms).
In a simple radar system configuration, for example, in commercial airborne weather radar receivers, the radar receiver amplifies both the reflected signal and the noise, but because the reflected signal amplitude is greater than the amplitude of the noise, the amplification magnifies the difference between the signal and the noise, facilitating differentiation between the noise and the reflected signal.
Signals received from the target are also filtered and analyzed for amplitude. The received signal is compared to detection thresholds. If the amplitude of the reflected signal exceeds the detection thresholds, the relevant portion of the signal is treated as potentially relating to a relevant target.
The effectiveness of this system is limited, however, by the characteristics of radar detectors and the signals they detect. In general, the signals received by the antenna and communicated to the receiver are subjected to display processing and automatic gain control (AGC) processing. Typically, the goal of the display processing is to improve the aforementioned signal-to-noise ratio by filtering noise, such as through use of any of a variety of processing techniques, for example an "M-out-of-N" processing scheme, where a display indication is made if M or more returns from N RF transmissions exceed the display threshold. On the other hand, typically, the goal of the AGC function is to control the number of false alarms displayed due to receiver noise. In this way, maximum receiver sensitivity can be ensured without the display of an excessive amount of false indications.
A typical prior art AGC system 100 is depicted in block diagram form as FIG. 1. As shown, an antenna 110 communicates signals received from the target (not shown) to a receiver 112. The output from receiver 112 is then compared to two different thresholds, namely a display threshold by a display comparator 114 and an AGC threshold by an AGC comparator 116. In this typical system, the result of the display threshold comparison is generally processed by a display processor 118, as previously noted, to enhance the signal-to-noise ratio. The result of the AGC threshold comparison, also once processed by an AGC processor 120, is typically used to directly control the receiver gain.
In controlling the receiver gain, competing constraints have heretofore dictated the use of a separate threshold for the AGC function and the display function, such as is illustrated in FIG. 1. Specifically, given that the average data rate in commercial airborne weather radar receivers is low, for example, on the order of one sweep over four (4) seconds, direct control of the false alarm rate with the AGC loop/function creates statistical instabilities due to the low false alarm rate which is equivalent to a low data rate for the AGC function. Accordingly, and with continued reference to FIG. 1, typically the AGC threshold is set to a lower value than the display threshold so as to achieve a higher false alarm rate. In this regard, the false alarm rate can be defined as the percentage of time the receiver output is above the AGC threshold. The AGC threshold is then adjusted such that a 50% false alarm rate yields a desired and acceptable displayed false alarm rate.
This control of the displayed false alarm rate, as previously noted, gives rise to maximum receiver sensitivity. However, it also raises various potential problems. For example, due to the dependence of the required threshold values on receiver linearity and dc offsets, over various temperature ranges, there can be some drift of the displayed false alarm rate. In general, the primary contribution to the false alarm rate, assuming a properly designed receiver, is thermal noise internal to the receiver, thus accounting for the aforementioned temperature dependency. While theoretically the thresholds, i.e., AGC and display thresholds, can be configured to be, and are substantially constant, a dc offset on the output of the receiver may upset the balance. That is, temperature variances which may be encountered through use of the radar system, may result in imbalances.
To avoid such a scenario, i.e., where an unacceptable false alarm rate would be encountered, in general, the receiver sensitivity is simply set lower than ideal to allow for a margin of temperature drift. While limited sensitivity is in and of itself not favorable, other problems also arise. Specifically, often the receiver sensitivity is adjusted through use of an iterative process in which temperature cycles and adjustments are made over anticipated temperature ranges. As will be appreciated by those skilled in the art, such an iterative process is costly, both in manufacture and repair.
Thus, there exists a long-felt and unresolved need for a radar system which addresses the foregoing disadvantages.